[isl-codegen]: Fix off by one in getNumberOfIterations

We need to remove one dimension. Any is correct as long as it exists. We have
choosen for whatever reason the dimension #dims - 2. This is incorrect if
there is just one dimension. For CLooG this case did never happen. For isl
however, the case can happen and causes undefined behavior including crashes.
We choose now always the last dimension #dims - 1. We could have choosen
dimension '0' but the last dimension is what we remove conceptionally in the
algorithm, so it seems better to actually program it that way.

While at it remove another piece of undefined behavior.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@174894 91177308-0d34-0410-b5e6-96231b3b80d8